1. Field of the Invention
This invention relates to a method and apparatus for placing in-ground piles of either crushed stone or rock, grout or concrete, or some combination thereof.
2. Description of Prior Art
It has been known in the prior art, to make pilings from concrete by casting them in place in the ground. This is accomplished by driving an elongate mandrel, i.e., a hollow tube of a selected cross-sectional area, into the ground, filling the resultant hole with fluid grout, and then pulling the mandrel back out of the ground leaving the grout in place to cure. The holding ability of such a piling comes from end-bearing and from the friction which is created by the close contact between the pile's sidewalls and the surrounding soil. It has also become standard practice to place a foot, aka, pile driving shoe, having a larger cross-sectional area than the mandrel, at the bottom of the mandrel before it is driven into the ground. The foot forms an open space around the mandrel. The hole created by the driving of the mandrel is filled with grout, i.e., cementitious fluid, stones, or rock. In the prior art grout may be gravity fed into and/or around the mandrel as it is being driven into the ground. See, e.g., Steding, U.S. Pat. No. 3,851,485 and Poma, U.S. Pat. No. 4,018,056.
A significant problem with the gravity feed method is that the grout often cannot flow into the hole fast enough to keep up with its formation, thereby resulting in voids. If soil or objects in the soil fall into these voids, the resultant pile diameter will neck down at the locations of the voids, weakening the pile. Other prior art devices have attempted to overcome this problem by pumping the grout into the mandrel under positive pressure as the mandrel is being driven. See, e.g., Hochstrasser, U.S. Pat. No. 3,084,518 and Federer, U.S. Pat. No. 4,618,289. However, pumping grout is also very problematic due to the inability to maintain a sufficient volumetric flow rate due to grout's the high viscosity. Accordingly, pumping is sometimes no more effective than gravity filling.
A twist on the gravity fed approach is seen in DeWitt, U.S. Pat. No. 4,992,002. In DeWitt, the mandrel is filled with grout prior to driving into the ground. A irretrievable steel foot is placed at the bottom the mandrel, which prevents the premature release of the grout. When the desired depth is reached, the mandrel is lifted out of the ground, leaving the foot behind and the grout flows out the bottom of the mandrel.
All of the aforementioned approaches are subject to the problem of necking, because no exterior reinforcing casing is provided. The grout will be still be subject to necking as the mandrel is withdrawn. Without an exterior reinforcing casing there is no reinforcement against side-loading from earth movements, wind and other influences. Accordingly, excess concrete or grout must be pumped at the top of the hole to prevent the collapse of the hole. This adds to the cost of the pile.
To provide the necessary reinforcement, the use of a full-length exterior reinforcing casing has been adopted in some circumstances. However, this is also less than ideal because the exterior casing is permanently left in the ground, thereby increasing the costs for each pile.
Moreover, the aforementioned prior art devices suffer from lack of durability. All of the prior art devices teach the use of pile driving a single mandrel into the earth. Accordingly, the mandrel must be made of very sturdy material such as heavy steel to withstand the blows of the pile driving hammer and the resultant reactionary forces of the earth. It is not uncommon for such mandrels, despite their sturdiness, to deflect during the driving process, which in turn will lead to inadequate pile formation or lost time. In time, such mandrels may have to be scrapped because they have lost their straightness or because they cannot withstand the blows of the pile driver. This is wasteful and expensive.
Additionally, the aforementioned devices do not accommodate the attachment of a vibrator without the need to halt the process. A vibrator is often used to assist in the delivery of pile forming material, such as stone, crushed rock or aggregate, or some combination thereof, into the resultant hole. The vibrator is clamped on to the mandrel and vibrates the mandrel. Prior to initiating the vibrator, the pile driving hammer must be lifted off of the mandrel to prevent damage to the pile driver. This results in additional delay in the pile forming process and consequently results in additional costs for the installation of the pile.
Also as can be seen in the above discussion, the foregoing prior art devices are specific for certain types of materials, for example, the DeWitt device can only be utilized for installing piles made of grout, but not stone. Accordingly, a pile forming enterprise must have access to separate apparatuses depending on the requirements of the job. This increases inventory costs, overhead and capital outlay.
What is needed is a single apparatus that is more durable when compared with single mandrel embodiments, that eliminates the need for an external casing that is left in the ground, that can be used in multiple piling forming situations, grout, stone, crushed rock, etc., and that can utilize a retrievable foot.